Detection of specific target analytes, or chemical compounds, is important for many applications, such as: detection of potentially harmful analytes in the environment, detection of analyte concentrations such that they do not exceed flammability limits, and early detection of chemical leaks. Target analytes may be liquids, vapours or gases, and are detected by sensors operating according to various detection mechanisms which are known in the art. A popular type of detection sensor is a sorption-based sensor (e.g., a polymer absorption sensor), wherein chemical sorption results in observable physical changes in the sensor. One example of such a sensor is a chemiresistor. A chemiresistor is a sensor where, upon sorption of specific chemicals, there is a physical change in the sensor, resulting in a corresponding change in sensor resistance (usually measured as the normalized change in resistance dR/R0; where dR is the change in resistance and R0 is the chemiresistor base resistance). Hereinafter, the term polymer absorption sensor (or “PAS”) will be used in the place of chemiresistor. In general, interest in these types of sensors stems from a number of factors such as their robustness, the fact that they are relatively cost-effective to manufacture, their ease of installation and minimal need for maintenance, whilst maintaining reliable output under a wide range of environmental conditions.
The sensitivity of a PAS to the concentration of a target analyte or to a confounding environmental condition (hereinafter “CEC”) is defined as the change in dR/R0 of the PAS in response to a corresponding change in the analyte concentration or in the magnitude of the CEC. To clarify the difference between the desired sensitivity to the target analyte from the undesired sensitivity to CECs, the term “sensitivity” is typically used when referring to the PAS response to the target analyte, whereas the term “cross-sensitivity” is typically used when referring to the PAS response to a CEC.
A CEC is an environmental condition which interferes with the accurate measurement of the concentration of the target analyte. In the context of PAS performance, the most important CECs are time-varying fluctuations in temperature and/or water saturation
PASs which can reversibly, reproducibly, and selectively detect hydrocarbon-containing vapours and liquids are of great interest in applications pertaining to the petrochemical industry. Current applications of PASs for chemical detection in the petrochemical industry include: detection of the leakage of volatile organic compounds (VOCs) during transport (pipelines, pump stations), storage (tanks), and extraction. PASs for the detection of VOCs have been known in the art since the early 1960's. There are numerous existing patents for PASs having applications in industries which include the transport industry, the petrochemical industry, and health and safety industries. Examples of such patents include U.S. Pat. Nos. 3,045,198; 4,224,595; 6,433,694; 7,112,304; and 7,138,090; and US Patent Applications 2006/0292033; 2007/0117207; 2008/0017507; and, 2011/0286889. In general, the focus of these involves improvements in sensor detection and sensitivity. These improvements were mostly made with respect to sensor materials (e.g., changes in conducting particle material, morphology, and polymer formulations) and electrical hardware.
More recent patent applications (for example, US 2007/0117207 and 2011/0286889) have moved toward reducing PAS cross-sensitivity to CECs. Cross-sensitivity to CECs causes undesirable changes in the dR/R0 of the PAS, thereby rendering it difficult or impossible to accurately interpret sensor measurements. Ideally, a PAS should be sensitive only to changes in the concentration of the target analyte, and should have zero cross-sensitivity to CECs.
Existing examples of chemical absorption sensors generally available to the petrochemical industry have one critical downfall: they exhibit a significant and highly-undesirable cross-sensitivity to CECs. This downfall is a result of the sensing mechanism utilized by these absorption sensors. PASs are chemiresistors, i.e., the electrical resistance of the sensor changes in response to changes in the immediate chemical environment. A typical PAS, as known in the art, is composed of an elastomeric polymer film (e.g., a polymer matrix composed of polydimethylsiloxane) which is affixed to a nonconductive substrate, such as a glass-epoxy circuit board. An electrical potential is applied across the polymer matrix to facilitate the measurement of PAS resistance. The polymer matrix will swell (expand), or increase in volume, while in the presence thermodynamically-compatible analytes, thereby inducing a detectable change in the electrical resistance of the polymer matrix. Changes in matrix volume can also occur in response to CECs, such as fluctuating temperature or water saturation. Temperature fluctuations will result in the polymer matrix expanding (increasing volume) or contracting (decreasing volume) with increasing and decreasing temperature, respectively, thus changing the sensor's resistance. Similarly, sensor water saturation will increase sensor volume through sorption of water, resulting in resistance changes in the sensor. Sensor cross-sensitivity to CECs is undesirable as this cross-sensitivity leads to false detections and inaccurate data, undermining the intended application of the device.
A typical example of a chemical sensor known in the art employs conductive particles which are distributed throughout the polymer matrix, wherein these particles serve to enhance changes in the resistance of the matrix when the volume of the polymer changes, thereby improving the sensitivity of the sensor. However, these types of sensors also exhibit an undesirable cross-sensitivity to CECs. In essence, any sensor film or matrix which relies upon physical changes resulting from absorption of a chemical analyte is generally also sensitive to volumetric and resistive changes which are dependent on temperature, water, or other environmental factors. A potential drawback of sensor cross-sensitivity to CECs is the likelihood of producing false positives and/or providing inaccurate data. Thus it is desirable, from an applications perspective, to improve sensitivity to target chemical analytes whilst minimizing cross-sensitivity to CECs, such as temperature and water saturation.
The present invention greatly improves upon the previous technologies in the art by incorporating at least one glassy and/or crystalline polymer within the polymer matrix. Such a glassy and/or crystalline polymer modifies the structure of the matrix, thereby yielding a sensor which mitigates cross-sensitivity to CECs.